1. Field of the Invention
The invention concerns components of tubular threaded joints which are used for example in petroleum applications and more precisely the lubrication of a portion or portions of a threaded element or elements of components of tubular threaded joints comprising a screwing abutment.
2. Description of the Related Art
The term “component” as used here means any element or accessory intended to be assembled by a thread to another component to constitute with that other component a tubular threaded joint. The component may be for example a tube of relatively great length (in particular about some ten meters in length), a tubular sleeve of some tens of centimeters in length, or an accessory for such tubes (a suspension device or hanger, a section-changing part or cross-over, a safety valve, a connector for a drill pipe or tool joint, and the like).
Such a component may be used for example for drilling or working a well. In that case the components are assembled together in order to be lowered into the hydrocarbons well or similar wells and to constitute a drill string, a casing string or a liner string or also a tubing string (working string).
The threaded elements produced at the end of a component (tube or sleeve) firstly have to be protected from corrosion during transport and storage on the drilling site and for that purpose they are traditionally coated with protective oils or greases on issuing from the manufacturing workshop.
At wells they may have to undergo a plurality of make up (tightening) and break out (untightening) operations. A make up operation is defined by a profile (or curve) which expresses the make up torque depending on the number of turns of rotation made. An example of a make up torque profile corresponding to a premium threaded joint with tapering threads is diagrammatically illustrated in FIG. 1. As can be seen, a make up torque profile can be generally broken down into four parts. A first part P1 during which the outer threads of the male threaded element (or “pin”) of a first component of a tubular threaded joint do not yet involve any radial tightening (interference fit) with the inner threads of the corresponding female threaded element (or “box”) of a second component of the same tubular threaded joint. A second part P2 during which the geometrical interference of the threads of the male and female threaded elements generates a radial tightening which increases in proportion to the progress of the make up operation (generating a low but increasing make up torque). A third part P3 during which a sealing surface at the outer periphery of the end part of the male threaded element interferes radially with a corresponding sealing surface of the female threaded element to produce a metal/metal seal. A fourth part P4 during which the front end surface of the male threaded element is in axial abutment with the annular surface of a screwing abutment of the female threaded element. That fourth part P4 corresponds to the terminal make up phase.
The make up torque CAB which corresponds to the end of the third part P3 and the beginning of the fourth part P4 is referred as the shouldering torque. The make up torque CP which corresponds to the end of the fourth part P4 is referred to as the plastification torque. Beyond that plastification torque CP it is considered that the male screwing abutment (end portion of the male threaded element) and/or the female screwing abutment (the zone disposed behind the annular abutment surface of the female threaded element) is the object of plastic deformation which can cause a deterioration in the sealing performances of the contact between sealing surfaces. The difference between the values of the plastification torque CP and the shouldering torque CAB is referred to as the torque on shoulder resistance and is identified as CSB (CSB=CP−CAB).
A tubular threaded joint is the object of an optimum tightening at the end of make up, which is the sign of an optimum mechanical strength of the threaded assembly, for example in relation to tensile forces but also accidental break out in a service situation, and optimum sealing performances. The designer of the threaded joint is thus induced to define, for a given type of threaded joint, an optimum make up torque value which, for all the assemblies of that type of joint, must be lower than the plastification torque CP (to avoid plastification of the abutments and the disadvantages which result therefrom) and higher than the shouldering torque CAB. Terminating a make up at a torque less than CAB does not in fact make it possible to guarantee correct relative positioning of the male and female elements and hence suitable tightening of their sealing surfaces against each other. The effective value of the shouldering torque CAB fluctuates greatly from one assembly to another for the same type of joint as it depends on the effective diameters of the threads and the sealing surfaces, both male and female, and it is appropriate for the optimum make up torque to be substantially greater than the shouldering torque CAB. Consequently, the greater the value of the torque on shoulder resistance CSB, the greater the margin that will be available to define the optimum make up torque and the more the threaded joint will be capable of withstanding the forces occurring in operation.
In order to protect the sensitive portions such as the threads from galling in the make up and break out operations the threads are traditionally freed of the protective grease and coated with special make up greases such as a grease in accordance with API RP 5A3 (formerly API Bull. 5A2). The use of such greases charged with heavy and/or toxic metals such as lead, in addition to the disadvantage of the need to perform a second coating operation at a well, suffers from the disadvantage of causing pollution of the wells and the environment as the grease excess is ejected from the threads during make up.
Other types of protection have been proposed.
Thus U.S. Pat. No. 6,933,264 proposes replacing the two successive coatings with grease by a single coating which is implemented at the workshop for manufacture of the threaded elements, using a thin layer of a lubricant of a pasty or waxy consistency (referred to as semi-dry), comprising at least one extreme-pressure additive with a chemical action.
That thin layer constitutes a semi-dry coating which suffers from the disadvantage of requiring mechanical protection from pollution by particles of dust or sand during transport and storage.
Other patent documents such as for example U.S. Pat. No. 4,414,247, U.S. Pat. No. 4,630,849, U.S. Pat. No. 6,027,145, U.S. Pat. No. 6,679,526 B2, US No. 2004/0166341 A1 and WO 2004/033951 propose replacing the greases by various protective coatings in the solid state which are applied at the workshop for manufacture of the threaded elements, and comprising a solid matrix which adheres to the substrate, in which there are dispersed particles of a solid lubricant or lubricants, among which molybdenum disulphide MoS2 is more particularly mentioned.
WO 2006/104 251 concerns a threaded joint having a viscous lubricating layer covered by a dry solid coating. This coating is not lubricating. The lubricating layer is not solid.
A protective coating in the solid state which causes much less pollution has also been proposed. It is formed by a lubricating matrix affording viscoplastic characteristics, in particular based on thermoplastic material, charged with particles of a solid lubricant or lubricants of at least two different classes. That coating resolves among others the problems involved in successive make up and break out operations, in particular under building site conditions, as well as the problems of protection from corrosion of the threaded elements. Moreover it affords high-quality lubrication. Unfortunately the applicants realised that, under conditions of use, in particular on a building site, the lubrication was so good that it no longer made it possible to tighten the male and female ends of a tubular threaded joint in conformity with the make up torque profile predefined for that tubular threaded joint.
In other words, that coating considerably reduces the amounts of friction at the screwing abutment at the end of make up under heavy Hertzian loads, referred to as “extreme pressure” loads, and at low frictional speeds, so that the plastification torque is reached much too early, and that gives rise to torque on shoulder resistance values which are much lower than the reference values obtained with the grease API RP 5A3. It may then be necessary to reduce the optimum make up torque value tabled for that type of joint and for the reference grease API and in those extreme cases the function of the abutment may no longer be guaranteed.
Therefore the aim of the invention is to improve the situation and more precisely to provide a lubricating (or coating) composition which affords a coefficient of friction which is so selected as to make it possible to achieve a selected torque on shoulder resistance value defined from that which was obtained for the same threaded joint with threaded elements coated with a standard grease API RP 5A3 in such a way as to make it possible to use a standard value for an optimum make up torque (standard value determined with a grease API RP 5A3).